The invention relates generally to the field of floor (minimum) level indicating circuits and more particularly to such circuits used for determining the noise level of an input signal in a receiver voting system. The term "floor level", as used herein, refers to the minimum magnitude of a signal and the term "noise level" refers to the background (residual) noise present on a signal.
Voting systems generally include several receivers which all receive the same transmitted signal and a selecting circuit which selects the received and detected signal having the lowest noise level. By maintaining the peak amplitudes of each of the detected signals at substantially equal levels, the selected signal will also correspond to the detected signal having the optimum signal to noise ratio. This optimum signal is then monitored until the selecting circuit determines that another receiver is receiving a signal having an even lower noise level. Thus the voting system continually selects the best (lowest noise level) received signal. Such systems are known in the prior art and one such system is described in a copending U. S. patent application entitled "An Improved Receiver Voting System", Ser. No. 604,562, filed Aug. 14, 1975, now Pat. No. 40,013,962, which is assigned to the same assignee as the present invention.
For each received signal a noise level indicative signal is generated. These indicative signals are then compared by the selecting circuit. Typically the noise levels of the signals received by different receivers will vary over an extremely large range, such as from 0 db to 40 db. If the noise indicative signals compared by the selecting circuit were linearly related to the noise level of the signal at the receiver, the selecting circuit would loose the ability to efficiently differentiate between two signals both having either very high noise levels or two signals having very low noise levels. This is because of the finite sensitivity of the selecting circuit and the requirement that the selecting circuit must be able to make decisions when the input noise levels vary over a large dynamic range. By way of example, the selecting circuit must be able to choose the best of two signals having noise levels of 0.5 .mu.V (microvolts) and 1.0 .mu.V. In a linearly related noise floor versus indicating signal voting system which must operate over the above-mentioned typical ranges, voting decisions would then be made even when noise levels of 5.0000 mV( millivolts) and 5.0005 mV were compared. Obviously such decisions are not needed or desired and therefore a linear system is not efficient. In addition, indicating signals would occasionally have very large magnitudes in a linear system and selecting circuits capable of handling these large signals would then be required.
A prior solution to this problem is to use a log amplifier followed by an envelope detector and a valley detector. The input audio signal is logarithmically amplified by the log amplifier, a waveform representing the envelope of this amplified signal is produced, and the minimum "valley" value of this envelope signal is determined. During an audio tone pause, the valley level of this envelope signal will be representative of the noise level of the incoming receiver signal.
A substantial disadvantage of this prior system is that the gain of the log amplifier is usually temperature unstable and this can lead to the control circuit selecting the wrong receiver signal to be monitored. Also logarithmatic amplifiers are complex circuits in which the logarithmic response is generally approximated by a piecewise nonlinear technique. Thus matching the gain characteristics of several logarithmic amplifiers becomes very difficult. In voting systems such gain matching must be extremely accurate or else the signal having the lowest noise level will not be selected.